1. Field of the Invention
The present invention relates to a belt driving device conveying a belt member related to image formation. More specifically, the present invention relates to belt units driving belt members such as an intermediate transfer belt, a transfer belt, and a photosensitive member belt, and an image forming apparatus equipped with the belt unit, such as a copying machine, a printer, or a printing apparatus. Further, the present invention is also applicable to a recording material conveying belt, and a fixing belt of a fixing device.
2. Description of the Related Art
In recent years, as a result of an increase in the processing speed of image forming apparatuses, a configuration has become mainstream in which a plurality of image forming units are arranged side by side with respect to a belt member to perform image forming processes in different colors in parallel. A typical example of such a belt member is an intermediate transfer belt in an electrophotographic full-color image forming apparatus. Toner images of different colors are successively transferred to and superimposed one upon the other on the belt surface, and the color toner images are collectively transferred onto a recording material. This intermediate transfer belt is suspended under tension between a plurality of suspension members, i.e., suspension rollers including a driving roller, and is capable of rotating. As is generally known in the art, such a belt member suspended under tension between a plurality of suspension rollers has a problem in that, during its running, the belt member may deviate toward either end of the rollers depending on the outer diameter precision of the rollers, the alignment precision between the rollers, etc.
As a solution to the problem of such deviation of the belt of a general nature, Japanese Patent Application Laid-Open No. 9-169449 discusses steering roller control using an actuator. Also, there is known a configuration as discussed in Japanese Patent Application Laid-Open No. 2001-146335, in which a belt deviation restriction member is provided.
However, the steering roller control as discussed in Japanese Patent Application Laid-Open No. 9-169449 requires a rather complicated control algorithm, and, further, it involves a rather high cost due to electrical components such as sensors and actuators. The configuration discussed in Japanese Patent Application Laid-Open No. 2001-146335 requires no sensors or actuators. However, there is a limitation to an increase in the processing speed of the image forming apparatus since the restriction member is constantly under the deviation force of the belt member during conveyance. Further, this configuration involves a high inspection/management cost in relation to the attachment precision of the restriction member.
In view of this, Published Japanese Translation of PCT Application No. 2001-520611 discusses a simple and inexpensive belt deviation control method in which a steering roller serving as a steering member automatically performs belt alignment through frictional force balancing (hereinafter referred to as the automatic belt alignment). The system as discussed in Published Japanese Translation of PCT Application No. 2001-520611 is equipped with a steering mechanism. More specifically, a steering roller composed of a central roller portion capable of following the rotation of the belt member and end members incapable of following the rotation of the belt member, is supported on a support stand rotatable with respect Co a steering shaft provided at the central portion. Here, the support stand is urged by a tension imparting portion compressed by a pressurization releasing cam, with the result that the outer peripheral surface of the steering roller imparts tension to the inner peripheral surface of the belt member.
The principle of the automatic belt steering will be described with reference to FIGS. 15A and 15B. As already described, end members 91 are supported so as to be incapable of following the rotation of the belt, so that they constantly receive frictional resistance from the inner peripheral surface of the belt member during belt conveyance.
FIG. 15A illustrates how a belt member 50, which is driven and conveyed in the direction of the arrow V, is wrapped around the end members 91 at a wrapping angle θs. As for the width (the direction perpendicular to the plane of the drawing), it is to be regarded as a unit width. A belt length corresponding to a minute wrapping angle component dθ of a certain wrapping angle θ will be considered; on the upstream side, i.e., the loose side, there is tangentially exerted a tensile force T, and, on the downstream side, i.e., on the tension side, there is tangentially exerted a tensile force T+dT. Thus, in a minute belt length, assuming that the force the belts exerts in the centripetal direction of the end members 91 is approximated as Tdθ, and that the end members 91 have a coefficient of friction μs, the frictional force dF is expressed by the following equation:dF=μsTdθ  (1)
Here, the tension T is controlled by a driving roller (not shown), and, assuming that the driving roller has a coefficient of friction μr, the following equation holds true:dT=μrTdθ  (2)That is,
                                          dT            T                    =                      -            μ                          ,                  d          ⁢                                          ⁢          θ                                    (                  2          ′                )            
When equation (2′) is integrated with respect to the wrapping angle θs, the tension T is obtained as follows:T=T1e−μrθ  (3)where T1 is the tension when θ=0.
From equations (1) and (3), the following equation is obtained:dF=μsT1e−μrθdθ  (4)
As illustrated in FIG. 15A, when the end members 91 move in the direction indicated by the arrows S, the wrapping start position (θ=0) has a deflection angle α with respect to the rotating direction. Thus, the downward component in the direction S of the force indicated by equation (4) is expressed as follows:dFs=μsT1e−μrθ sin(θ+α)dθ  (5)
                              F          s                =                              μ            s                    ⁢                      T            1                    ⁢                                    ∫              0                              θ                s                                      ⁢                                          ⅇ                                                      -                                          μ                      r                                                        ⁢                  θ                                            ⁢                              sin                ⁡                                  (                                      θ                    +                    α                                    )                                            ⁢                              ⅆ                θ                                                                        (        6        )            In this way, the downward force in the direction of the arrow S that the end members 91 receive during belt conveyance (per unit width) is obtained.
FIG. 15B corresponds to a plan view of FIG. 15A as seen from the direction of the arrow TV. Suppose belt deviation occurs leftward when the belt member 50 is conveyed in the direction of the arrow V as illustrated in FIG. 15B. At this time, as illustrated in FIG. 15B, only the left-hand side portion has an overlapping width w between the belt member 50 and the end member 91.
More specifically, the left-hand side end member 91 receives a downward force in the direction S of Fsw, and the right-hand side end member 91 receives a downward force in the direction S of zero. It can be explained that this difference in frictional force between the end portions constitutes a motive power generating a moment FswL around the steering shaft (In the assumption in FIG. 15B, the left-hand side, i.e., the deviation side, is directed downwards). In the following, the moment around the steering shaft will be referred to as the steering torque.
The direction of the rudder angle of the steering roller 97 generated through the above principle corresponds to the direction in which the deviation of the belt member 50 is restored to normal, so that it is possible to perform automatic steering.
However, in a configuration in which the width of the belt member is larger than the width between the friction portions at both ends, when the belt member deviates to one end side, steering is started by inclining the steering roller. In this process, the following problem is involved.
As a result of the inclination of the steering roller, the belt member, which has deviated to one end side, starts to move toward the other end side. When, as a result of the movement of the belt member, the friction member at the other end starts to be brought into contact with the belt member, due to the twisting of the belt member caused by the inclination of the steering roller, the frictional force between the friction member at the other end side and the belt member is larger than that when the steering roller is not inclined. As a result, the moving force of the belt member is reduced, so that it is impossible to effect smooth deviation adjustment on the belt member.
Such a problem can also arise when the steering roller is inclined in the case in which the belt member is in contact with the friction portions at both ends.
Thus, to further smoothen the movement of the belt member, it is desirable to reduce this resistance force.